Isolated switched-mode direct-current (DC) to DC power converters use a transformer to convert power from an input source into power for an output load. Such power converters include power switches that convert DC input power into alternating current (AC) power that is fed to the primary side of the transformer. AC power supplied on the secondary side of the transformer is rectified and filtered so as to provide DC power to the output load. The primary-side power switches are typically controlled by pulse-width-modulated (PWM) control signals. A controller generates the PWM control signals with a frequency and duty cycle that are appropriate to meet the power needs of the output load.
The controller typically uses a linear closed-loop feedback technique to maintain the output voltage near a desired target. The controller may be implemented using analog or digital circuitry, and may be located on the primary or secondary side of the power converter. So as to maintain the integrity of the isolation barrier of the power converter, any signals crossing between the primary and secondary sides must pass through isolators, e.g., transformers, opto-couplers. Isolated switched-mode power converters increasingly use digital controllers that are located on the secondary side, so as to avoid passing the output voltage, which must be sensed by the controller for linear closed-loop feedback control, through an analog isolator at the primary-to-secondary boundary. Furthermore, locating the controller on the secondary side allows for ready communication between the controller and components of the output load, e.g., for load management, without requiring isolators.
However, some techniques used by the controller may require information regarding the input voltage or current of the primary side of the power converter. For example, a linear feedback control technique may be augmented with feedforward control techniques so as to quickly compensate for input voltage transients. However, feedforward control techniques require use of the input voltage, or an estimate thereof, from the primary side. Similarly, the controller may need to detect primary-side fault conditions, which also requires information regarding the input voltage or current. The input voltage or current may be directly sensed by a secondary-side controller, but this requires an analog isolator which is preferably avoided. Alternatively, the input voltage or current may be estimated based upon the output voltage or current, which may be sensed on the secondary side of the power converter. However, the output voltage is low-pass filtered, typically by an output capacitor and the load resistance. The delay incurred by the low-pass output filter means that changes in the input voltage are only detectable in the output voltage after a considerable time lag. Such a lag may make use of the output voltage unfeasible for purposes of feedforward control and/or detection of primary-side faults.
A rectified voltage is typically available on the secondary side of an isolated power converter prior to the output filter, i.e., between the transformer and the output filter. Input voltage transients may be detected in the rectified voltage without incurring the delay of the output filter. Hence, the rectified voltage may be used to estimate the input voltage with only a minimal delay. The rectified voltage may also be used for other purposes, including estimating the magnetic flux of the transformer and measuring the time delay between issuance of PWM control signals and corresponding pulses in the rectified voltage.
To meet such goals, circuits and methods are needed that are capable of accurately sensing secondary-side voltages, including a rectified voltage, which may be rapidly changing. Such sensing should be power efficient and the associated circuitry should be feasible for implementation within a digital controller on the secondary side of an isolated power converter